Q: Grease pencil, how "Anti-aliasing Threshold" works?
There was a bug with Anti-aliasing  in pre 2.9 versions. Are you up to date?
If you're experience flickering lines when moving the camera or an object then this is a known bug. I believe the fix at the moment is render in double the size and then compress. This works for me but naturally takes significantly longer to render.
Thanks
Matt